The invention relates to a method for testing ceramic socket inserts of hip-joint endoprostheses.
Currently, hip-joint endoprostheses consist, as a rule, of modularly constructed systems. A metal shaft with a pin, on which a spherical head is placed, is anchored in the femur. The spherical head articulates against a socket or a socket insert. A socket is implanted directly in the femur, whilst a socket insert is first inserted in a socket housing which is then anchored in the pelvic bone.
In addition to components for endoprostheses of the hip joint made of metal and plastics material, there are also components that are made of high-purity, high-density ceramic material. These components, present significant advantages, in contrast to components made of different materials, such as complete biocompatibility and maximum wear resistance. Doubts exist, however, regarding the mechanical strength of such components, since ceramic materials are brittle, which means that instances of non-homogeneity of the material, for example micro-cracks, represent an increased risk of fracture in the event of loading. The reliability against defects resulting from components that are risk-attendant can be increased if these components are successfully detected and eliminated out by means of a suitable test after they have been produced. It is not, however, possible to detect components that are risk-attendant with any certainty by means of the usual non-destructive testing methods, for example X-ray testing, ultrasonic testing or dye-penetration methods.
For these reasons, method have been developed with which it is possible to test, in particular, components of hip-joint endoprostheses that are made of ceramic materials. For example, a method for testing ceramic sockets or socket inserts of hip-joint endoprostheses is known from DE 197 18 615 A1, in which a force is allowed to act on the inner surface of the latter in such a way that all the volume elements of the socket or socket insert respectively that are under a load, when physiologically loaded, become loaded and stresses are generated thereby that are higher, by a defined factor, than the stresses that are generated in the case of the physiological load.
In the case of the known overload tests, the so-called proof tests, carried out on ceramic socket inserts, the difficulty lies in generating the same stress ratios during the test that prevail in the case of a socket insert that is inserted in a socket housing, which in turn is implanted in a hip joint, and is loaded with a spherical head.
It is not possible to insert the socket inserts into the socket housings for testing purposes in order to be able to carry out a proof test as a quality control after production. After testing, it would be impossible to remove the socket inserts from the respective socket housing again without damage. Moreover, on account of the manufacturing tolerances of socket inserts and socket housings, reproducibility of the contact ratios between the components would not be guaranteed. This is necessary, however, since in a proof test it must be possible to repeat the most unfavourable case of stress distribution caused by the contact of the components.
The object of the present invention is therefore to propose a testing method with which a distribution of stresses is attained in the socket inserts comparable to those stresses that occur in loaded socket inserts which are inserted into a socket housing which in turn is implanted in the pelvic bone.
The object is achieved by introducing the testing force into the socket insert by way of a predetermined sub-area of the functional surface, which symmetrically surrounds the pole of the inner functional surface of the socket insert, and by applying a supporting force that counteracts the testing force simultaneously to a sub-region of the outer surface of the socket insert.
In the testing method in accordance with the invention, a supporting force acts on a sub-region of the outer surface of the spherical cap-shaped socket insert at the same time as the action of the testing force. During testing, the supporting force is increased as the loading by the testing force increases. The testing force is introduced by way of a predetermined sub-area of the inner surface of the socket insert, the inner spherical cap, which symmetrically surrounds the pole of the socket insert. As a result, stresses are generated in the socket insert in the same way as stresses also occur in an actual case of loading in the socket insert. The whole outer surface of the socket insert with conically shaped regions is displaced during testing under sufficiently uniform tensile strain.
The size of the sub-area of the functional surface of the socket insert, upon which the testing force acts, is predetermined by the height of the section which is covered by the sub-area. The height of the section is to amount to approximately 15% to 30% of the maximum inside diameter of the functional surface of the socket insert. Such is the height of a spherical section that goes right into the socket insert and by way of which the testing force is mechanically introduced into the socket insert by means of a die or the space in which the testing force is applied by means of a pressure fluid under pressure that, as a result of the predetermined height of the section, a loaded area is created that renders possible optimum simulation of the actual loading of a socket insert.
The testing force increases linearly up to a predetermined maximum value which is to be attained within approximately 10 seconds. Like the testing force, the supporting force likewise increases linearly and amounts to approximately 10% of the testing force. The testing force is introduced perpendicularly in the direction of the pole of the functional area. The supporting force is introduced in the opposite direction. To this end, for example, a die can press against the outer surface of the socket insert, with it being possible to generate the supporting force hydraulically, pneumatically or with springs.
It is advantageous to use, as a basis in each case, the maximum diameter of the functional areas of the socket inserts as a criterion for the level of the testing forces. For example, in the case of a socket insert having a maximum diameter of the functional area of 28 mm and a service life of 20 years that is to be guaranteed, a testing force of 13 to 15 kN is selected.
If the testing force is introduced mechanically by means of a spherical section that is arranged at the end of the die, the material of this section is not to damage the functional surface of the socket insert during testing. Moreover, it must be guaranteed that manufacturing tolerances are compensated for. For this reason, it is advantageous if at least one surface layer of the section consists of a material that is softer than the material of the socket insert and has a module of elasticity of approximately 300 to 1500 MPa. Plastics materials, in particular polytetrafluoroethylene, have proved to be suitable.
The socket inserts are held in the holding support of the testing device in accordance with the invention in a so-called receiving ring. The receiving ring supports the socket inserts in the edge region of the maximum outer periphery. The wall of the opening of the receiving ring extends, so as to be adapted to this wall region, in a conical manner. Whilst the outside diameters of the receiving rings are constant, the diameters of the receiving openings can be different in accordance with the outside diameters of the socket inserts. As a result, advantageously it is possible to be able to test socket inserts with different diameters of the functional areas in one and the same receiving arrangement, merely by changing the receiving rings.
In a further advantageous development of the invention, is a narrow ring made of a ductile material laid between the receiving ring and the socket insert. The ring can, for example, be made of copper or another soft metal or even of a suitable plastics material. With the aid of this ring of ductile material it is possible to reconcile manufacturing tolerances and compensate for possible structures, for example surface roughness. Furthermore, when the testing force is acting, friction-associated, uneven jamming and tilting of a socket insert inside the receiving ring are avoided. The withdrawal of a socket insert out of the receiving ring and the subsequent removal of the ring of ductile material are possible without a tool and thus without the risk of damage to the socket insert.